1. Field of the Invention
The present invention relates generally to earth-boring bits of the rolling cutter type and to improvements in gage and heel row compacts for such bits by which the resistance to wear is increased, the improved compacts being formed with a hard metal jacket and a superabrasive working surface.
2. Description of the Prior Art
Wear-resistant inserts or compacts are utilized in a variety of earth-boring tools where the inserts form rock cutting, crushing, chipping or abrading elements. In rotary well drilling, some geological formations are drilled with bits having cutting structures of wear-resistant (usually sintered tungsten carbide) compacts held in receiving apertures in rotatable cones. In such bits, there is usually on each cone a group of cylindrical compacts that define a circumferential heel row that removes earth at the corner of the bore hole bottom. Further, it is common to insert additional cylindrical compacts, called "gage" compacts, on a "gage" surface that intersects a generally conical surface that receives the heel row compacts. These gage compacts protect the gage surfaces to prevent erosion of the metal of the cones that supports the heel row compacts. As a result, fewer heel compacts are lost during drilling and the original diameter of the bit is better maintained due to decreased wear. Moreover, the gage compacts also ream the hole to full "gage" after the heel compacts are worn to an undersized condition.
Fixed cutter bits, either steel-bodied or matrix, are also utilized in drilling certain types of geological formations effectively. While these bits do not feature rotatable cones, they also have wear-resistant inserts advantageously positioned in the "shoulder" or "gage" regions on the face of the bit which are essential to prolong the useful life of the bit.
A typical prior-art wear-resistant insert was manufactured of sintered tungsten carbide, a composition of mono and/or ditungsten carbide cemented with a binder typically selected from the iron group, consisting of cobalt, nickel or iron. Cobalt generally ranged from about 6 to 16% of the binder, the balance being tungsten carbide. The exact composition depended upon the usage intended for the tool and its inserts.
In recent years, both natural and synthetic diamonds and other superabrasive materials have been used, in addition to tungsten carbide compacts, as cutting inserts on rotary and fixed cutter rock bits. In fact, it has long been recognized that tungsten carbide as a matrix for superabrasives has the advantage that the carbide itself is wear-resistant, fracture-tough, and offers prolonged matrix life. U.S. Pat. No. 1,939,991 describes a diamond cutting tool utilizing inserts formed of diamonds held in a medium such as tungsten carbide mixed with a binder of iron, cobalt, or nickel.
In some prior-art cutting tools, the superabrasive component of the tool was formed by the conversion of graphite to diamond. U.S. Pat. No. 3,850,053 describes a technique for making cutting tool blanks by placing a graphite disk in contact with a cemented tungsten carbide cylinder and exposing both simultaneously to diamond forming temperatures and pressures. U.S. Pat. No. 4,259,090 describes a technique for making a cylindrical mass of polycrystalline diamond by loading a mass of graphite into a cup-shaped container made from tungsten carbide and diamond catalyst material. The loaded assembly is then placed in a high temperature and pressure apparatus where the graphite is converted to diamond. U.S. Pat. No. 4,525,178 shows a composite material which includes a mixture of individual diamond crystals and pieces of precemented carbide.
U.S. Pat. No. 4,148,368 shows a tungsten carbide insert for mounting in a rolling cone cutter which includes a diamond insert embedded in a portion of the work surface of the tungsten carbide cutting insert in order to improve the wear resistance thereof. Various other prior art techniques have been attempted in which a natural or synthetic diamond insert was utilized. For instance, there have been attempts in the prior art to press-fit a natural or synthetic diamond within a jacket, with the intention being to engage the jacket containing the diamond within an insert receiving opening provided on the bit face or cone. These attempts were not generally successful since the diamonds tended to fracture or become dislodged in use.
This lack of success is attributable to the boring mechanics of rolling cone bits. Unlike other applications for superabrasives, inserts used in rolling cone bits are subjected to extreme transient, or shock, force loads during drilling. Superabrasives are generally extremely hard but extremely brittle, and cannot withstand extreme transient loads without cracking or other brittle failure. It is believed that such brittle failure can be avoided by securing the superabrasive to a substrate formed of a fracture-tough material. The fracture-tough material then can absorb the shock loads that the superabrasive is incapable of withstanding alone.
Provision of a superabrasive with a fracture-tough, shock-absorbing substrate does not provide the final solution: there remains the problem of retention of the superabrasive on the substrate. U.S. Pat. No. 4,148,368 discloses a diamond insert imbedded in a fracture-tough insert to be interference fit into a rolling cone cutter of an earth-boring bit. That disclosure suggests that the diamond be affixed to the remainder of the insert by an interference fit or brazing. Interference fitting of a diamond into a insert, with the insert, in turn, interference fit into a socket on a rolling cone is unsatisfactory because the diamond is incapable of withstanding the residual stress of the initial and subsequent interference fits upon exposure to the transient force loads of drilling.
Simply brazing a diamond or other superabrasive also is unsatisfactory. Diamonds, as well as other superabrasives, often contain impurities in their crystal lattices that render the materials thermally unstable; that is, subject to cracking and other deformation and decomposition upon heating. Additionally, superabrasives have among the lowest coefficients of thermal expansion of known materials. Therefore, upon the heating and cooling present in brazing operations, a superabrasive will expand and shrink less than most any material to which it may be brazed and the braze material itself. The different shrinking rates of superabrasives and the substrate and braze materials cause residual thermal stresses in the superabrasive that can cause the superabrasive to crack upon cooling, or upon exposure to the transient loading of drilling.
The former problem largely has been solved by the relatively recent development of TS (thermally or temperature-stable) grades of superabrasives. These TS superabrasives are processed to remove the impurities that cause cracking upon heating of the superabrasives. However, the latter problem still remains an obstacle to brazing or infiltrating superabrasives to a fracture-tough substrate.
Still further, brazing a superabrasive element alone yields unsatisfactory results apart from thermal decomposition and deformation problems. Braze materials appear to be incapable of wetting or otherwise successfully bonding to the surfaces of superabrasive elements. Thus, the retentive strength of brazed superabrasives is limited to the shear strength of the braze material, which generally is low and certainly incapable of withstanding forces encountered by rolling cone earth-boring bits in drilling operation.
Other solutions have been attempted. U.S. Pat. No. 4,604,106 discloses a compact for use in earth-boring bits having diamond particles sintered with cemented carbide particles to form a composite insert. Such an insert is unsatisfactory, however, because its wear resistance is limited to that of the cemented carbide that binds the particles together: at the working surface of such an insert a substantial amount of cemented carbide is exposed along with the diamond particles. Such an insert does not exhibit the wear-resistant properties of an insert having a working surface comprising entirely or primarily superabrasive. It is at least theoretically possible to form such a composite insert having a working surface primarily of diamond, but the extremely high-pressure sintering and pressing processes required to form such an insert are extraordinarily expensive.
U.S. Pat. No. 4,493,488 discloses superabrasive inserts affixed to fracture-tough substrates for use in fixed cutter, or drag bits. U.S. Pat. No. 5,049,164 discloses another superabrasive insert having a superabrasive affixed to a fracture-tough substrate, for use in fixed cutter, or drag bits. The inserts disclosed are not adapted for the rigorous environment encountered by rolling-cone earth-boring bits.
There continues to exist a need for improvements in compacts of the type utilized as wear-resistant inserts in earth-boring bits, particularly in the gage and heel regions of rolling cone bits, which will improve the useful life of such bits.
A need also exists for improvements in the wear-resistant inserts used in such bits, whereby such inserts are provided with improved abrasion resistance and diamond retention characteristics.
It is advantageous, therefore, to provide an insert for use in an earth-boring bit of the rolling cone variety having an abrasion-resistant working surface formed primarily of a superabrasive, such as polycrystalline diamond, which is affixed to a fracture-tough substrate by a relatively low-cost, low pressure and temperature process.